Polysulfide paper pulping composition



United States Patent O 3,551,286 POLYSULFIDE PAPER PULPING COMPOSITIONRichard G. Barker, Princeton Junction, NJ., assignor to Union CampCorporation, New York, N.Y., a corporation of VirginiaContinuation-impart of application Ser. N o. 620,288, AMar 3, 1967. Thisapplication Jan. 3, 1968, Ser.

No. 695,495 The portion of the term of the patent subsequent to Sept.30, 1986, has been disclaimed Int. Cl. D21c 3/00 U.S. 'Cl. 162--82 6Claims ABSTRACT OF THE DISCLOSURE An aqueous, alkaline modified kraftpulping solution for Iobtaining increased cellulose yield in paperpulping, while minimizing corrosion and air pollution, comprising atotal sulfur content of from to 60 grams per liter, said sulfurconsisting of from 43 to 80 percent from an alkali metal polysulde, from40 to 15 percent from an alkali metal sullide, and up to 17 percent froman alkali metal thiosulfate, particularly a pulping solution, the activeinorganic constituents of which are recirculated for additional pulping,after recovery.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of my copending application, Ser. No. 620,288,filed Mar. 3, 1967, for Sodium Polysulde Pulping Process andRegeneration, now Pat. No. 3,470,061.

BACKGROUND OF THE INVENTION The kraft process is the most extensivelyemployed for the pulping of lignocellulosic materials. The activeingredients used for digestion in the kraft process are sodium hydroxideand sodium sulfide in a strongly alkaline, aqueous solution. Almost anywood species, for example, slash pine, can be pnlped by this process,

The total sulfur content of the kraft pulping, or white, liquor, isgenerally between about 5 and 60 grams per liter. Most often, andpreferably, it is in the range of from to 30 grams per liter. This whiteliquor is usually diluted with water, or spent pulping liquor, beforeaddition to a charge of wood chips and the mixture of wood chips andpulping liquor is then heated, under pressure, at a temperature of 150C. to 190 C. for digestion.

The kraft pulps show Wide variations in the lignin content, and thecellulose yield is different for each such lignin content. It isobviously desirable to increase the pulp yield, for any given lignincontent, as this means that an increased amount of pulp is derived froma given wood charge. Polysulde pulping has previously been disclosed asa way to improve this yield. Inclusion of polysulde sulfur in lthe kraftwhite liquor increases the carbohydrate retention of the wood charge,and thereby increases the pulp yield. This has been previouslydemonstrated, for example, in U.S. Pat. No. 3,216,887-Landmark and U.S,Pat. No. 2,944,928-Kibrick et al.

However, an increase in the polysultide content of the white liquor,though improving the yield, has meant an increase in the total sulfurcontent of the white liquor. Further, in pulping, the polysulde isconverted to sulde so that the spent liquor has a higher concentrationof sulfide than would otherwise be true. If the inorganic constituentsof the liquor are then recirculated, after recovery, for additionalpulping, and the polysulde concentration is again increased, the totalsulfur content is increased still further, leading to severe problems.

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As is well known, increased sulfur content increases corrosion of pulpprocessing equipment and, perhaps more importantly, greatly increasesthe sulfur losses to the atmosphere, contributing drastically to airpollution.

The solution to the problem, and an object of this invention is toprovide a pulping solution, particularly one where the inorganicconstituents can be recovered and recirculated, where the polysuldecontent is increased so as to increase the pulp yield, while maintaininga total sulfur concentration within acceptable limits as to corrosionand pollution. Such a composition is not described in either of the twopreviously mentioned prior art patents, nor can such a composition beattained by following the disclosures in those patents.

SUMMARY OF THE INVENTION In accordance with the present invention, anaqueous pulping solution is employed having between 5 and 60 grams perliter of sulfur, more preferably between 10 and 30 grams per liter.While the higher total sulfur range can be employed within the presentinvention, limiting the maximum sulfur content to 30 grams per literavoids excessive corrosion and pollution problems. The desired pulpingaction is not obtained with sulfur concentrations below about 5 gramsper liter.

As important as the total sulfur content, is the source of this sulfur.According to the present invention, from 43 to 8O percent of the totalsulfur content is obtained from an alkali metal polysulde, preferablysodium polysultide. The sodium polysulde is preferred because of itssignificantly lower cost and easier recovery. More preferably, from 50to 70 percent of the total sulfur content is derived from the alkalimetal polysulde.

. Between about 40 and l5 percent of the total sulfur should be derivedfrom an alkali metal sulfide, As sodium polysulde is the preferredalkali metal polysulde, the preferred sulfide is sodium sulfide. Morepreferably, the percentage of sulfur obtained from the alkali metalsulfide is between about 40 and 17 percent.

The third major sulfur-containing constituent of the system is an alkalimetal thiosulfate, generally sodium thiosulfate because of theconstituency of the other two sulfur-containing materials. While thealkali metal thiosulfate has no real function in the pulping process, itresults from the use of the other materials and is, preferably,maintained at as low a level as possible. According to the presentinvention, this level is generally less than about 17 percent, andpreferably less than about 10 percent.

The pulping solution must also contain a certain amount of alkali. Theamount of alkali contained in the pulping solution is not critical andthe solution is effective for pulping with, for example, a total alkalicontent of from l5 to 400 grams per liter, as Na2O. More preferably, thetotal alkali content is from 70 to 120 grams per liter. This alkali isgenerally derived from the group consisting of sodium hydroxide, sodiumcarbonate, and sodium sulfide.

BRIEF DESCRIPTION OF THE DRAWINGS The figure is a plot of yield comparedto Kappa number showing the improved yields obtained with a compositionof the present invention as opposed to those of a prior art composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In my aforementioned copendingapplication, Ser. No. 620,288, filed Mar. 3, 1967, I have described aprocess for oxidizing a white liquor or white liquor-black liquormixture in order to convert the alkali metal sulfide, particularlysodium sulfide, to an alkali metal polysulfide, particularly sodiumpolysulfide. According to that application, various solid, inorganicmanganese oxidizers can be employed, including Mn203, Mn3O4, MnS2,partially oxidized MnO' MnOOH, and MnO2, and the oxidation takes placeat a temperature in the range of from 40 to 120 C. The pulpingcompositions of the present invention can be maintained, recirculatingthe active inorganic constituents of the pulping solution, by employingthe process of that aforementioned application. By that process, thepolysulfide level can be brought within the desired range withoutincreasing the alkali metal thiosulfate level beyond that desired andwithout the addition of alkali metal polysulfide. Thus, the total sulfurcontent is not raised beyond the desired level, ameliorating thecorrosion and pollution problems.

Employing the pulping composition of the present invention,significantly improved yields are obtained. This is true with respect tothe standard kraft composition, as well as by comparison with the priorart processes.

These improvements will be described in greater detail in the followingexamples. These examples should be considered as illustrative only, andnot as limiting in any way the full scope of the invention as covered inthe appended claims.

EXAMPLE 1 In this example the pulping composition of the presentinvention was employed, and compared with a standard kraft pulpingcomposition, on slash pine to obtain a pulp with a Kappa value of about100. The compositions of the two pulping solutions, with respect tototal sulfur content, are shown below, both as grams per liter of sulfurobtained from the particular component, and as to the sulfur percentageobtained from each sulfur-bearing component.

TABLE I Sulfur in Sulfur in pulping composikraft pulping tion of thepresent composition invention Grams Percent Grams Percent Sulfur-bearingper of total per of total component liter sulfur liter sulfur Sodiumpolysulfide 0 7. 52 46 Sodium sulfide 16. 4 100 6. 08 37 Sodiumthiosulfate 0 0 2. 8 17 Employing these two compositions to pulp slashpipe obtained from the same batch, with an active alkali content in thepulping solution, in each case, of about 100 grams per liter, expressedas NazO, and using a digesting temperature of 160 C., the yield with thecomposition of the present invention was approximately 2 percentgreater, and with a lower Kappa value. Specifically, the standard kraftcomposition gave a 56.8 percent yield with a Kappa number of 111, whilethe process employing the composition of the present invention resultedin a 58.5 percent yield, with a Kappa number of 101. The sodium sulfideconcentration in the black liquor was 7.8 grams per liter for the kraftprocess and 8.1 grams per liter for the process employing thecomposition of the present invention. Thus, the yield improvementobtained with the composition of the present invention was gainedwithout a significant increase in the final sulfur content, and theattendant problems of increased corrosion and pollution.

EXAMPLE 2 A comparison, similar to that in Example 1, was made betweenthe composition of the present invention and a kraft pulping compositionin producing a pulp with a Kappa number of approximately 130. Thepulping com- 4 positions as shown in the table below, in the same manneras in Table I of Example 1:

A yield improvement of about 4 percent was realized in this example, thekraft composition providing a 60.3 percent yield of pulp with a Kappanumber of 125, while the composition of the present invention showed a64.6 percent pulp yield at a Kappa number of 135. Again, the pulpingprocesses were carried out under similar conditions, each of thecompositions having about 100 grams per liter of `active alkali,expressed as NaZO, with a digesting temperature of 160 C.

The kraft composition black liquor showed 6.5 grams per liter of sodiumsulfide, while employing the composition of the present invention, ablack liquor was obtained having a 4.8 grams per liter of sodiumsulfide.

Thus, at both Kappa levels, when the composition of the presentinvention is compared with a standard kraft pulping composition, yieldsare improved without a correlated rise in the black liquor sodiumsulfide concentration, so that corrosion and pollution problems are notincreased along with the yield increase.

By comparison, the employment of the polysulfide composition of theprior art which, as previously mentioned,

provided yield increases, also resulted in significant increases in thesodium sulfide level of the black liquid, representative of increasedpollution and corrosion. This is illustrated in Example 3.

EXAMPLE 3 In this example a standard kraft composition was cornparedwith a modified kraft composition where the kraft liquor containedsufficient polysulfide for an application of 1 percent polysulfidesulfur on the wood. These compositions are shown in Table III in thesame manner as were the kraft composition and composition of the presentinvention in Table I of Example 1.

TABLE III Sulfur in Sulfur in kraft pulping modified kraft compositioncomposition Composition forlpercent Grams Percent Grams Percentapplication of per of total per of total polysulfide sulfur liter sulfurliter sulfur Sodium polysulfide 0 0 8. 8 37. 3 Sodium sulfide 14. 8 10014. 8 62. 7

Employing essentially the same conditions with the two pulpingcompositions at a digesting temperature of 173 C., the straight kraftcomposition yielded 57.8 percent pulp with a Kappa number of 99.3, whilethe modified composition yielded 59.0 percent pulp with a Kappa numberof 97.5. Thus, there was a yield increase of a little over 1 percent.However, the sodium sulfide content of the black liquor 'was increasedsignificantly, in fact, almost by a factor of 2. The content of sodiumsulfide in the black liquor from the straight kraft composition was 6.6grams per liter, while from the modified process it was 11.2 grams perliter. Employing this modified composition, not only would there besignificantly increased corrosion, and significantly increased airpollution, but the cost of the process would additionally be raised dueto the requirements for additional sulfur, including the lossesresulting from such a high sulfur content in the black liquor.

EXAMPLE 4 In this example, the composition of the present invention wascompared with that of Landmark, where as significant percentage of thepulping composition sulfur is obtained from sodium polysulfide, butwhere that percentage is significantly lower than the percentages calledfor in the present invention. Similarly, in the Landmark composition thethiosulfate percentages are significantly higher. A composition wasprepared according to the Landmark patent disclosure having 37 grams perliter of sodium sulfide, grams per liter of sodium thiosulfate, and 7.28grams per liter of polysulfide sulfur. The composition had an effectivealkali content of 74 grams per liter, 88.7 grams per liter of activealkali. A composition lwas also prepared having 74 grams per liter ofeffective alkali and 33.8 grams per liter of sulfur, all obtained fromsodium sulfide. This latter composition lwas then oxidized, employingthe process of my aforementioned copendiug application, particularlyusing manganese dioxide at a temperature of 60 C. The resultingcompositions, which were used for pulping, are as follows, withdesignations the same as those of Table I in Example l:

The above-mentioned compositions were employed to pulp slash pine chips,taken from the same batch, employing an impregnation time of minutes andtemperature of 110 C., and the same maximum pulping temperature of 160C.

' As can be seen from the figure, at equivalent Kappa numbers, theyields were significantly increased employing the composition of thepresent invention, as opposed to the composiiton of the Landmark patent.IFor example, at Kappa numbers between 40 and 45, the composition of thepresent invention gave pulp yields of betweeen and 56 percent, while theLandmark composition showed pulp yields between 531/2 and 541/2 percent.

Thus, employing the composition of the present invention, with the sametotal sulfur content as in a prior art composition with differentconstituent percentages, significantly increased yields are obtained.These increased yields are gained without an increase in corrosion andpollution problems, as the sulfur contents of the resulting blackliquors are essentially the same.

The active inorgnaic constituents in the black liquor resulting from thecomposition of the present invention in this example can be recovered innormal fashion and the polysulfide content regenerated by treatment withthe manganese oxidant described in my aforementioned copendingapplication. These inorganic constituents can then be recirculated foradditional pulping. Thus, the pulping liquor composition can bemaintained at approximately the levels shown in the table, adding onlysufficient materials to make up for that lost through normal processing.As with standard pulping solutions, the composition of the presentinvention can also contain sodium carbonate and sodium sulfate, withoutinterfering with normal processing.

The invention has been described in detail with particular referencemade to various preferred embodiments thereof, but it will be understoodthat variations and modiications can be effected within the spirit andscope of the invention as described hereinabove and as defined in theappended claims.

I claim:

1. A sulfur-containing aqueous alkaline modified kraft pulping solutionfor the digestion of lignocellulosic materials for paper-making having atotal sulfur content of from 5 to 60 grams per liter, said sulfurconsisting of (a) from 43 to 80 percent from an alkali metal polysulfide(b) from about l5 to 40 percent from an alkali metal sulfide, and

(c) less than about 17 percent from an alkali metal thiosulfate.

2. The liquor of claim 1 containing from 15 to 400 grams per liter oftotal alkali, as N20, said total alkali being derived from sodiumhydroxide, sodium sulfide, and sodium carbonate.

3. The liquor of claim 2 having from 70 to 120 grams per liter of totalalkali.

4. The liquor of claim 1 wherein said alkali metal is sodium and saidtotal sulfur content is from l0 to 30 grams per liter.

5. The liquor of claim 4 wherein said sulfur consists of from about 50to 75 percent as sodium polysulfide, vfrom about 17 to 40 percent assodium sulfide, and less than about 10 percent as sodium thiosulfate.

6. The liquor of claim 1 having from 70 to 120 grams per liter of totalalkali, wherein said alkali metal is sodium and said total sulfurcontent is from 10 to 30 grams per liter and wherein said sulfurconsists of from about 50 to percent as sodium polysulfide, from about17 to 40 percent as sodium sulfide, and less than about l0 percent assodium thiosulfate.

References Cited UNITED STATES PATENTS 3,216,887 5/1964 Lanomarr-162--38 3,470,061 9/1969 Barbor 162-38 S. LEON BASHORE, PrimaryExaminer R. H. ANDERSON, Assistant Examiner U.S. Cl. X.R.

Disclaimer Junction, N J. POLYSULFIDE 3,551,286.Rz'oharcl G. Barker,Princeton PAPER PULPING COMPOSITION. Patent dated Dec. 29, 1970.Disclaimer filed Aug. 1, 1973, by the assignee, Union Camp Corporation.Hereby enters this disclaimer to claim patent.

s 1 through 6, inclusive, of said [Official Gazette December 5, 1.973.]

